(i) Field of the Invention
The present invention is directed to the preparation of synthetic amino acid polymers and , more particularly, is directed to the use of these polymers to protect chemical compounds, especially drug substances, from degradation, and to release the compounds under specific conditions.
(ii) Description of Related Art
Under physiological conditions, proteins (polymer chains of peptide-linked amino acids) normally do not exist as extended linear polymer chains. A combination of molecular forces, including hydrogen bonding, hydrophilic and hydrophobic interactions, promote thermodynamically more stable secondary structures that can be highly organized (helices, beta pleated sheets, etc.). These structures can then combine to form higher order structures with critical biological functions. Natural proteins are peptide-linked polymers containing 20 different amino acids, each with a different side-chain. The details of the folding into higher order structures are dependent on the type, frequency and primary sequence of the amino acids in the protein. Since each position in the polymer chain can be occupied by 20 different amino acids, the thermodynamic rules that describe the details of protein folding are complex. For example, we are currently unable to design a synthetic protein with a substrate-specific enzymatic site that is predicted by the primary amino acid sequence. More complete discussions of the structure and function of proteins are found in Dickerson et al. “The Structure and Action of Proteins” Harper and Row, New York, 1970 and Lehninger “Biochemistry” Worth, New York, 1970, pp. 109-146.
However, some basic rules of protein folding have been discovered. In general, the side chains of the 20 L-amino acids commonly found in natural proteins can be placed in two categories, hydrophobic/non-polar and hydrophilic/polar, each playing separate roles in protein conformation. In the standard “oil drop” model for protein folding, the amino acids with more hydrophobic side chains (Val, Leu, Phe, Met, IIe) are sequestered to the inside of the protein structure, away from the aqueous environment. Frequently, these hydrophobic side chains form “pockets” that bind molecules of biological significance. On the other hand, hydrophilic amino acids (e.g. Lys, Arg, Asp, Glu) are most frequently distributed on the outer surface of natural proteins, providing overall protein solubility and establishing a superstructure for the internalized hydrophobic domains.
A highly preferred conformation found in many natural proteins is the 3.613 alpha-helix. This right-handed helix contains 3.6 amino acids per turn and is stabilized by hydrogen bonding (about 3 kcal/mol) involving the amide hydrogen and a carbonyl oxygen, separated by 13 atoms along the backbone of the polymer chain. Since the amino acid side chains in the alpha-helix point away from and perpendicular to the helix axis, any of the amino acids (except Pro) can participate in the helix. Other structures can also appear in higher order protein conformations, including the 310 helix, and the important left-handed, three residue helix found in collagen and pleated sheets.
Other amino acids can also be used with predictable results in the preparation of synthetic proteins. Tyrosine (Tyr) is frequently found internalized, with its 4-hydroxy hydrogen, hydrogen-bonded to another amino acid or potential ligand/enzyme substrate. Thus, Tyr can be utilized to produce hydrophobic pockets with a potential for hydrogen bonding. Proline (Pro) has been found to be sufficient, but not always necessary, for a sharp turn in the peptide chain, allowing for cooperative interactions of different sections of the same polymer. At higher polymer concentrations, Pro can also disrupt helical structure, producing a “less organized” protein. Cysteine can be utilized to stabilize higher order structures by linking polymer chains through high energy (about 50 kcal/mol) disulfide bonds. Some amino acids do not have distinct hydrophobic or hydrophilic character and provide a “place-keeping” function or contribute more subtle effects on the overall protein structure.
Some work on synthetic polypeptides has proceeded with the goal of producing textile products with desirable properties, but the technology has been largely too expensive to compete with natural products, and with other synthetic polymers. In the pharmaceutical industry, work on synthetic polypeptides has focused again on specific amino acid sequences having intrinsic hormonal or drug activities. A more complete discussion of the use of synthetic polymers for textiles and pharmaceuticals is provided by Block in “Polymer Monographs” Gordon and Breach, Vol. 9, 1983. A historical perspective is provided by Watson “Molecular Biology of the Cell” W.A. Benjamin, Inc., New York, 1970.
Thus, it should be apparent that to date there has been in the art only a limited ability to synthesize proteins with a view to achieving a final product having a particular selected property.